American swimmers question why seven of their Russian counterparts are allowed to compete despite initially being banned following allegations of a massive state-sponsored doping operation in their homeland.

He was referring to Sun testing positive for a prohibited stimulant in 2014 and then serving his three-month suspension in secret.

Chinese fans have reacted to news of Ms Chen’s failed drug test with shock and disbelief.

Some postings on the popular Sina Weibo microblogging site say they will wait for more details on the case before commenting, while others are directing their anger towards Australian swimmer Mr Horton.

“It feels like there’s a big conspiracy Horton!” writes user -zezhen.

“I expect a lot of the domestic and foreign media will come and provoke,” reads another post by user Evanesce.

Ms Chen was scheduled to compete in the 50 metre freestyle race on Friday.

The Chinese Swimming Association says Ms Chen has applied to the International Olympic Committee for further tests and a hearing.